Wilhelm Wundt discovered the time it took for people to become consciously aware of sound. His study is considered to be the first ... observational study, psychological experiment, psychological perspective, theological discovery

I believe the answer is: psychological experiment

In this experiment, Wilhelm Wundt became the first person that implement scientific method in analyzing the pyschological phenomenon.

Due to this result, Wilhelm Wundt also able to make psychology to be acknowledged as a separate scientific field from biology.